Correct me if I'm wrong, but I believe that a tsunami only occurs in the Pacific, whereas a tidal wave occurs in the Atlantic. At least that's always what I believed.
But then again, I was always led to believe that
hurricanes only occured in the Atlantic, whereas out west they had to worry about
typhoons. Much to my surprise, it seems that the western gulf of Mexico now gets hurricanes, too.
People change the meanings of words to suit them. "Tsunami" just sounds so much cooler than plain old "tidal wave". It's like salsa